Python Modulus Giving String Formatting Errors

Question

I'm trying to perform the modulus of a value in python, but I'm getting errors as it's interpretting the modulus as a string formatting constant, from my knowledge. My initial guess would be to type cast this, but then it hangs.

    val = pow(a,n,p)
    val = y1*val
    val = val % p

Are the two lines of code corresponding to this question. Right now, when I run this, I get: TypeError: not all arguments converted during string formatting At the second line.

If I wrap val into an integer and type cast it...it takes extremely long to calculate.

I'm not too skilled with python, my guess is I'm missing something simple, but what?

what's y1? Assuming it's a string, val = y1*val also produces a string. It's probably helpful to print repr(val) after each step for debugging. — Wooble
– Wooble, Commented Mar 20, 2012 at 18:02
Sure the error occurs on the second line? It would make perfect sense in the third line, given that y1 is a string (which would result in val also being a string in the last line). — Niklas B.
– Niklas B., Commented Mar 20, 2012 at 18:06
What pow function is that with 3 arguments? Also, can you give more information on where these arguments come from? val should already be a number, not a string, so I'm guessing there is something fishy going on upstream. — Collin Green
– Collin Green, Commented Mar 20, 2012 at 18:08
@Keeyai: the default pow function has three arguments. Type "help(pow)" at the interpreter. — DSM
– DSM, Commented Mar 20, 2012 at 18:09
@DSM: I see! So, pow(x,y,z) is equivalent to (xy) % z -- so you are effectively doing (((an) % p) * y1) % p. I'm still banking on weird arguments upstream. Check the types of everything and see what is coming out. By the time it throws your error, val is a string containing multiple % signs. — Collin Green
– Collin Green, Commented Mar 20, 2012 at 18:16

jsbueno · Accepted Answer · 2012-03-20 18:13:27Z

6

If yu are getting this error, y1 itself is a string. You can't perform numeric calculations with strings - when you do "int(y1) " - it is not casting, it is converting the number represented by characters inside the string o an actual numeric value - and that is the only way you can perform numeric operations on it.

If it is takin thta log, it is probable because you are trying convert "y1 * val" to int - which is wrong already - if y1 is a string, "y1 * val" gives you y1 concatenated to itself "val" times - so it would be a really huge number. You need to have the value i n "y1" as a number before multiplying - as in:

val  = int(y1) * val

answered Mar 20, 2012 at 18:13

jsbueno

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1 Comment

r36363 Over a year ago

This was the issue, I somehow stripped the int(y1) i had prior to the lines of code listed after reading it in from a file.

Steven Rumbalski · Accepted Answer · 2012-03-20 18:22:09Z

2

As you can see from this code, the % operator has different meanings with strings than with numbers.

>>> 1 % 2
1
>>> '1' % 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting

My guess is that y1 is actually a string. Here's the difference the type of y1 makes:

>>> val = 10
>>> y1 = '2'
>>> val * y1
'2222222222'
>>> y1 = 2
>>> val * y1
20

answered Mar 20, 2012 at 18:22

Steven Rumbalski

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